Method for cleaning contaminated water

ABSTRACT

This invention refers to an original solution for the purification of contaminated water streams with the activated sludge method, involving: a) a two-stage aeration system operating at high rate and low rate conditions respectively, where the first stage operates in anoxic conditions promoting denitration, whereas the second stage operates under oxidant conditions so that nitrates are removed to an extent of 70%; b) a system for moving the biological sludge, where the sludge produced in the first aeration tank is totally rejected, and that produced in the second aeration tank is recycled to the first. By disposing of the sludge from the first aeration tank, 70-80% of the phosphate content are removed and the system is relieved of toxicity; c) a system using special ion exchanging resins enclosed in a casing, which are inflatable in water by a factor of 200-350, which have a high ion exchange capacity, and which bind the total metal ions presents; d) in the course of the biological treatment, the thermal activation of the biological culture is introduced by recycling the culture to the first aeration tank, where conditions of toxicity related biological stress develop on the culture, so that thermal activation and selectivity of species are promoted in it; e) following the above, the installation for cleaning contaminated water operates to specification with a very high and steady yield, achieving water purification to an extent of BOD reduction by 95% and with the option for the application of tertiary purification with the development of aquatic plants, leading to the production of recycled water of BOD&lt;5 and hardness &lt;10 in a simple and cost-effective manner. The biological treatment process described constitutes a method for optimizing water purification installations operating with the activated sludge method. It is most suitable for use with installations located in warm climate regions where the oxygen supply in the aeration tanks is reduced due to the high temperature of the contaminated water and where the need for recycling water is maximal.

Our invention refers to a method for cleaning contaminated domestic andindustrial effluent water streams with the active sludge method and bythe introduction of innovative processes for treating the ammoniate andphosphate contents of such streams and for the promotion of theactivation of biological culture. The methods achieves a high degree ofpurification at a rapid rate. It results in obtaining recycle water of ahigh purity, using low installation and operating cost equipment.

The activated sludge method for cleaning contaminated water has a verywide range of applications and is often simply referred to as the methodfor biological treatment of contaminated water. It employs a tank forprimary settling, an aeration tank and a tank for secondary settling. Inthe course of its development, multi-stage systems have been introduced,where two- or three- stage aeration is conducted. The two- stageaeration method is combined with the development of conditions for thegrowth of bacteria only in the primary settling tank and for the growthof higher micro-organisms (such as protozoa and other) in the secondarysettling tank. This has also required the adjustment of the residencetimes in the to tanks. Furthermore, multi-stage systems for thetreatment of ammoniates and phosphates present in the contaminated waterstream have been developed.

The process of biological treatment of contaminated water with theactivated sludge method is characterised by the residence times in highrate processes and low rate processes. The high rate process, with aresidence time of 2.5 hours appears to favour the growth of bacteriaprincipally, whereas the low rate process favours the growth of avariety of micro-organisms where bacteria and protozoa are the principalconstituents, depending on the residence times, usually being in therange of 4 to 6 hours. The high rate process has a small oxygenconsumption and maintains a small concentration of a biological culturein the order of 500 kg./liter. It responds positively to a rise intemperature up to 45° C., achieves high degrees of purification, up to70%, and requires a small consumption of energy. The low rate process,however, does not benefit from rising the temperature above 20° C.,operates in an excess biological culture concentration of 3000 kg/litter and has high operating costs associated with it.

It is also known that in warm-climate regions, the sewage or industrialwaste end-up in aeration tanks where temperatures in excess of 30° C.prevail. Thus, air solubility in such tanks is limited. The saturationoxygen content at 15 Celsius is 10.4 kg/lit, whereas this drops to 6.6kg/lit at 35 Celsius. Thus, extended aeration installations tend tounderperform in such regions.

It is derived from the analysis above that the method for contaminatedwater purification by the activated sludge method is not effectivelystudied nor optimised as yet. Among functionality problems associatedwith it, one could list:

it has particular adaptation problems when the temperature of thecontaminated water stream rises.

as sewage and industrial waste are complex materials always carryingtoxicity, in the form of organic toxic matter and heavy toxic metals, toa higher or lesser degree, the approach is rather of theoreticalimportance

the toxicity has a variable composition and intensity and inhibits thedevelopment of the biological culture and the rate of the biologicaltreatment, thus leading to serious economic and environmental drawbacks.

Taking into account the above mentioned advantageous and disadvantageousaspects of the biological treatment of contaminated water with theactivated sludge method, the inventors have invented, researched andoperated pilot solutions which manage to handle all these aspectssuccessfully and highly beneficially.

BRIEF DESCRIPTION OF THE DRAWING

A pilot plan diagram of the instant invention.

The inventors have proved that the process of biological treatment ofcontaminated water with the method of the activated sludge can beoptimally operated in a two-stage aeration process in a process, wherethe first stage operates in a non-acidic environment with removal ofnitrates and acting as a toxicity trap. According to this solution, thebiological culture in the first aeration tank also acts as an adsorberof heavy metals and toxicity. This culture is totally removed and is notrecycled into the system, Following disposition of the sludge from thefirst aeration tank, phosphate removal is also accomplished, to a degreein excess of 70%. Active sludge is developed and maintained in thesecond aeration tank which operates in highly oxidant conditions bypromoting, the oxidation of ammoniates to nitrates and by recycling thesludge for providing for sludge requirements in both aeration tanks.

During the purification process involving two-stage aeration, the sludgefrom the second stage is only employed. This sludge is entered into thefirst aeration tank which contains toxicity, thus resulting to thedevelopment of competition between micro-organisms for their survival.Thus, the most resilient and active of bacteria which express the higherenergy levels of micro-organism development, prevail. These operatingconditions are defined as thermophilic activation of the biologicalculture. This thermophilic activation will develop to a steady state forthe whole purification system, where in the first tank developmentconditions will involve competition of species by the application oftoxic stress, whereas in the second aeration tank seeds of activatedbiological culture will be conveyed and optimum conditions for thedevelopment of biological sludge will prevail.

The combination of a high rate and a low rate process developed andoperating as a toxicity trap is an original and optimised solutionapplicable to the warm climate regions. It is also a highly original andsimple solution leading to a high degree of nitrate and phosphateremoval. It has been shown that the rate of nitrate removal in a highrate process conditions, developing in a non-acidic environment with acontinuous recycle of a water load with sludge from the second, low rateprocess tank (where oxidant conditions prevail leading to nitrateformation) is in the order of 60-80%. In parallel, as the biologicalsludge from the first aeration tank is totally rejected, a high fractionof the phosphates (60-80% of total contents) is also received.

The inventors have also invented, elaborated and operated solutions forthe toxic relief of the installations for cleaning contaminated waterwith the activated sludge method, by introducing means for the selectivebinding of the heavy metals supplied and for the introduction of primarysettling conditions for the reduction of the pollutant load in the tank.

The heavy metals in the form of ions are bound to ion-exchanging resinsof a special type, invented and operated by the same inventors. Suchresins are positioned onto a bed where from the contaminated waterpasses. The ion exchanging resins are polymer to macroplegmaticstructure of an Mc value of 50,000 conversion products. Sulfonic andcarboxylic groups have been inserted to these resins to a maximumdensity to ion exchange ratio near the theoretical 5.6-6.2 limit. Suchresins are inflated in water 250-350 fold their weight, and, having lowporosity, they bind rapidly the total of metal ions passing through,carried by the water.

The ion exchanging resins mentioned above are added to the aerationtanks. As they are inflated 250-350 fold in water, they bind the solublemetal ions, whereas due to their low porosity the sludge cannot flowthrough.

By the addition of alkaline water, the pH in the primary settling tankis regulated to a value of 8.3. Under such conditions it has been shownthat the largest proportion of the suspended material settles, leadingto a reduction of environmental load in the order of 40-50%, with atotal binding of phosphates by the calcium ions present. Thus,conditions for reducing the pollutant load in cases of intensiveoperation of the biological treatment plant are developed. Alkalinewater is formed, by the treatment of the sludge in an anaerobicdigestion process in the thermophilic region. This process is documentedin patent 90117377/3/10 filed by the same inventors.

The improvements to the contaminated water treatment method of activatedsludge proposed by this invention lead to the operation of such plantsin an effective, trouble-free manner, achieving high rates ofpurification, a reduced operating cost and operation according tospecifications.

The installations for cleaning contaminated water as developed by thisinvention achieves the full removal of toxicity, a high rate ofevolution, and a low operating cost. It can operate without a primarysettling tank, especially when the solids suspended in the contaminatedwater stream have a concentration inferior than 5000 kg./litter. Theprocess as a whole achieves purification to a degree of BOD reduction inthe order of 98% and can be combined with a unit for the development ofaquatic plants where auxiliary nitrate, phosphate and pollutant removaldevelops. Such a combination has a small installation and operation costand can achieve the production of recycle water meeting high purityspecifications and a quality biological evolution. In general, what hasbeen described and developed in this invention is a modern approach forthe evolution of installations for contaminated water purification whichhas a highest economic, scientific and environmental significance.

EXAMPLE NO 1

In the pilot plant of the attached drawing, which is located at the endof the central sewage duct of the Greater Athens area, and which is fedat a flowrate of 20 lit/h., the effectiveness of the water cleaningprocess was investigated: The contaminated water stream was fed with aBOD load of 320+/-100, a suspended solids concentration of 520+/-140containing heavy metals at a concentration of 200 kg./lit, ammoniates at100 kg./lt., and phosphates at a quantity of 60 kg./lt. The pilot unitoperated for 45 days with no problems at the following operatingparameters:

Suspended solids in the first aeration tank: 480+/-50 kg./litter

Suspended solids in the second aeration tank: 5300+/-150 kg./litter

Aeration in the first aeration tank: 1-1.1 cu.m./h

Aeration in the second aeration tank: 1.4-1.7 cu.m./h

The outlet stream had a BOD load of 30+/-5 and a COD load of 45+/-10. 65+/-5% of heavy metals were bound in the first aeration tank, nitrateremoval at 82% was achieved, whereas phosphate binding by the rejectedsludge was in the order of 72%.

EXAMPLE NO 2

A second pilot unit involving a single aeration tank of a size equal tothe total of the two tanks of the first unit was operated for thetreatment of a supply of 20 litters/hour, as well, The relativeperformance of the two units was investigated in terms of BOD reductionat the outlet.

Results are summarised below:

    ______________________________________                                               Pilot Unit Second pilot unit involving                                        of drawing attached                                                                      single stage aeration                                       ______________________________________                                        24 hrs    60 ±/- 10                                                                              180 +/- 50                                              48 hrs   40 +/- 4     150 +/- 60                                              72 hrs   25 +/- 5     150 +/- 60                                              ______________________________________                                    

EXAMPLE NO 3

In the pilot unit illustrated in the drawing attached, a quantity of 50gram of ion exchange resin was placed in a casing suspended in theaeration tank. This resin was inflatable in water up to 300 fold. with adehardening ability of 5.2 degrees. The result of this addition was thetotal elimination of metals during the experiment. Furthermore, thebiological culture in the second tank developed without any disruption,resulting in a significant improvement of the rate and the degree ofpurification. During the 25 days of operation of the unit at conditionssimilar to those of experiment 1,the outlet BOD was 15+/-5 with a steadyyield since the first day of operation and an air consumption of 2.3 cu.m./minute.

Following the end of the experiment, the ion exchange resins exhibited areduced ion exchange ability. They were removed, reactivated bysubmersion in salted water (sea water) and reset to operation. At theseconditions, the influence of the residence time on the degree ofpurification was investigated with the following results:

    ______________________________________                                        Flowrate, lit/h.                                                                          Outlet BOD                                                                              Air Consumption, cu.m./h.                               ______________________________________                                        20          18 +/- 5  2.2                                                     23          20 +/- 6  2.3                                                     26          20.5 +/- 6                                                                              2.3                                                     30          19.8 +/- 8                                                                              2.3                                                     ______________________________________                                    

It was found that with an increase in the flow by 50%, the cleaningsystem is adapted to optimum operating conditions and that the degree ofpurification remains steady.

EXAMPLE NO 4

In the pilot unit illustrated in the attached Drawing, fed at a flowrateof 20 litters/hour at a contaminated water stream composition of theexperiment no 1, the influence of the rate and time of recycle on thedegree of purification in the two aeration tanks was investigated;

    ______________________________________                                        Aeration tank No. 1:                                                          Sludge production: 3750 +/- 200 kg./h.                                        Quantity and rate of sludge recycle                                                    Own sludge % from tank 2                                                                             Outlet BOD                                    ______________________________________                                        1-48 h.  100%          0        290 +/- 10                                    1-48 h.   50%          50%      220 +/- 20                                    1-48 h.   0           100%      140 +/- 10                                    1-48 h.   0           100%      290 +/- 10                                    ______________________________________                                        Aeration tank No. 2:                                                          Sludge production: 4000 +/- 300 kg./h.                                        Outlet BOD                                                                              aeration, cu.m./h.                                                  ______________________________________                                        160 +/- 8 3.8              Note a)                                            130 +/- 6 3.2              Note b)                                             35 +/- 5 2.7              Note c)                                             25 +/- 5 2.2              Note d)                                            ______________________________________                                    

a)After 5 hours of operation the aeration tank does not exhibitbiological activity

b)The operation of the aeration tank exhibits reduced biologicalactivity

c)The sludge in the aeration tank has some activity and a light colour

d)Following placement of a casing with ion exchange resins in the secondaeration tank.

EXAMPLE NO 5

The effluent water from the purification installations with a BOD of20-100 is passed to a tank for the development of aquatic plants(aquatic hyacinths, aquatic celery, etc.) operating at greenhouseconditions covered with a transparent polymer which permits the sun-raysto pass. The aquatic plants develop very rapidly and the final outflowwater has a BOD less than 2 and a hardening factor of 10. Thus, it is ahigh quality recycle water. The aquatic plants can be cut andtransferred to the anaerobic digestion unit or be used for making animalfoodstuffs.

We claim:
 1. A method for biologically treating contaminated water usinga two stage aeration system which comprises a first aeration tank and asecond aeration tank comprising the following steps:a) in the firststage, introducing contaminated water into the first aeration tank wherethe aeration tank has an anoxic environment, and a resulting treatedsludge is removed from the system, and about 70-80% of the phosphate andnitrate content of the contaminated water is removed; b) in the secondstage, water from the first stage is introduced into the second aerationtank which operates under oxidizing conditions which produces nitrates,and a resultant treated sludge is recycled and excess sludge isintroduced into the first aeration tank, wherein either or both aerationtanks contain ion exchanging resins which are inflatable in water by afactor of 200-350 and bind soluble metal ions.
 2. The method accordingto claim 1, where a toxic material supplied to the system is adsorbed bya rejected sludge of the first aeration tank which is fed with thesludge recycled from the second aeration tank and conditions ofbiological selectivity are created with a continuous activation of thebiological culture.
 3. The method according to claim 1, where the secondaeration tank operates at conditions for thermophilic activation ofmicro-organisms, wherein said conditions are free of heavy metals in thepresence of ion exchanging resins which are inflatable up to 350 fold inwater.
 4. The method according to any one of claims 1, 2, and 3, wherethe introduction of contaminated water or toxic material into the firstaeration tank results in a varied intensity of toxic stress to thesludge, wherein conditions for species competition is created and steadyfunction of the system is maintained.
 5. The method according to claim 1where the purification system evolves towards steady state operation,wherein a selected thermophilic bacteria from the first aeration tank inthe form of grains or biologically activated complexes are transferredto the second aeration tank.
 6. The method according to any one ofclaims 1, 2, 3 and 5, whereina) the first aeration tank operates underanoxic conditions resulting in the conversion of nitrates to nitrogen,b) the second aeration tank operates in oxidant conditions resulting inthe conversion of ammoniates to nitrates which are continuously recycledwith the sludge to the first aeration tank which results in the bindingof 70-75% of a nitrogen containing material, and c) 60-75% of phosphatesare removed with disposal of the sludge from the first aeration tank. 7.The method according to claim 1, where the activation of a biologicalculture is promoted in parallel by a thermophilic selection of a rangeof thermophilic microorganisms, and the rate and the effectiveness ofthe water cleaning system is increased.
 8. The method according to claim1, where if the contaminated water contains a high proportion of metalions, said metal ions are bound by the introduction of the inflatableion exchanging resins in the contaminated water or in a primarysettlement tank of the system.
 9. The method according to claim 1,whereina) the first aeration tank acts as a toxicity trap and nitrate isremoved, b) sludge produced in the first tank is rejected after 60-75%of the phosphate content is removed, c) active sludge from the secondaeration tank is recycled to the first aeration tank, which results in acompetition for survival of microorganisms which are activated, d) athermophilic biological culture is developed in the second aerationtank, and e) the heavy metal ions are bound with the special ionexchanging resins which are inflatable in water by a factor of 300 andare placed in the second aeration tank.
 10. The method according toclaim 1, wherein the ion exchanging resins have a Mc value of 50,000conversion products and contain sulfonic and carboxylic groups insertedto a maximum density to ion-exchange ratio of about 5.6-6.2.